Such a mechanism is for example described in patent application EP 1,351,104, which describes the use of at least one retractable tooth supported by a program wheel, the retractable tooth being associated with a wheel whereof the teeth act as cam surfaces. However, the retractable tooth is a sliding element that is not kept in a given position and that could therefore inopportunely position itself in the active position in case of impact. This would risk causing blockages of the system or breaking the clockwork movement.
Such a mechanism is also described in patent application EP 2,490,084, which describes the use of a retractable tooth mounted pivotably between an active position, in which it is driven by the clockwork movement, and an inactive position, in which it is not driven by said clockwork movement. However, an elastic element is provided making it possible to keep the pivoting retractable tooth in the inactive position. The use of such an elastic element as a return spring is not optimal. In fact, the behavior of this type of part with age is not known. Additionally, in case of rupture or deformation of the elastic element, the mechanism risks no longer working correctly. Furthermore, the retractable tooth, which moves by pivoting, then has an asymmetrical shape. As a result, the retractable tooth will not be able to engage with a wheel having a different pitch diameter, much less in a bidirectional manner, in light of that asymmetry.
Another mechanism for driving a date indicator using a retractable tooth is for example described in patent EP 1,240,559. In this mechanism, to deal with leap years, the use of a retractable tooth is provided subject to a spring to move it and keep it in an inactive or retracted position and kept in the active position for example by an eccentric having a truncated cylinder shape.
Such a mechanism using the action of a spring is not optimal. In fact, in document EP 1,240,559, the spring is mounted against the inner wall of a months wheel. This construction creates friction, which may be quite significant, thereby generating a significant loss of torque. Additionally, the behavior of the spring with age is unknown. The risk is that after a certain amount of time, the retractable tooth will retract incorrectly or be kept in position incorrectly, which could cause poor operation of the system over the long-term. The same is true in case of impact, if the spring becomes deformed. The mechanism then risks no longer working correctly.
One aim of the present invention is therefore to offset these drawbacks, by proposing a springless drive mechanism for managing the entry and exit of the retractable teeth, that is easy to implement, and generates little friction so as to use less energy than the traditional systems.
Another aim of the present invention is to propose a drive mechanism allowing a two-way rotation of the indicator.
Another aim of the present invention is to propose a compact drive mechanism, comprising a reduced number of components and withstanding impacts.